This invention pertains generally to the field of plastic packaging and particularly to thermoformed plastic containers.
Reclosable plastic containers are used for the packaging of a wide variety of products which will be held in the container and utilized by the consumer over a period of time. The containers may be utilized to enclose and protect products that are periodically used, such as a compact disk, a videotape, an audio tape, etc., or to contain multiple products which are used by the consumer one at a time, e.g., diapers, paper napkins, disposable wipes, screws, fasteners, etc. The cover for such reclosable containers is typically connected to a receptacle by a hinge about which the cover rotates when opened by the consumer to permit access to the interior of the container.
Various production processes are used to form plastic containers. Injection molding is well suited to the formation of relatively thick-walled, rigid plastic containers of the type that are intended for long-term use by the consumer. A variety of hinge structures are possible in injection molded containers because of the flexibility offered by the injection molding process. However, injection molding is not economically well suited to the production of plastic containers intended for use in the packaging of relatively low cost products. The molds used in injection molding processes are expensive, and the injection molding process itself is a relatively slow production process involving complex equipment which must be carefully monitored and controlled.
For the production of relatively low cost plastic containers for use in the packaging of bulk or relatively inexpensive consumer products, the thermoforming process is more widely used. In the thermoforming process, a thin sheet of thermoplastic is held over a mold and is heated to a temperature at which it can be plastically deformed. The heated plastic is then drawn using vacuum and pressure into the mold where it conforms to the surfaces of the mold, cools, and hardens to retain the shape of the mold. The formed product can then be die cut from the surrounding sheet, removed from the mold, and a new section of plastic sheet advanced into place over the mold. This thermoforming process, and variations on it, can be carried on in a continuous production process, allowing very high production volumes and low unit costs. These processes are used to form the common xe2x80x9cblister packsxe2x80x9d in which a product is sealed between the formed plastic xe2x80x9cblisterxe2x80x9d and a removable panel, commonly of paperboard. Such blister packaging is relatively inexpensive and is typically discarded after the package is opened by the consumer.
Reclosable hinged plastic containers may also be made by the thermoforming process. Typically, the receptacle of the container and the cover are formed integrally from the same sheet of plastic and are joined together by a so-called xe2x80x9cliving hingexe2x80x9d which flexibly joins the cover and the receptacle. The plastic of the living hinge is sometimes thinned during the forming process to make it more flexible. While such integrally formed reclosable containers can be made very economically, and are extensively used in the packaging of a wide variety of products, the integral cover and receptacle structure has certain inherent limitations. Generally, the cover and receptacle must be formed of the same plastic. Although it is possible to thermoform containers having a different plastic material for the cover and the receptacle, by utilizing a sheet of two separate plastics joined together, the choice of plastics that can be used is limited and the requirement for special plastic sheet stock makes the process less economical. Furthermore, some types of common plastics are not well suited to be formed with a living hinge because of the inherent rigidity or vulnerability to fatigue failure of the plastic material. Polystyrene is an example of a type of plastic commonly used in packaging but not well suited to the use of an integral living hinge. Containers formed with living hinges may also suffer from a bias or memory in the hinge which tends to draw the cover partially open. In certain situations, it may be disadvantageous to utilize containers with integrally formed covers, for example, where the cover is to be printed, labeled or decorated after forming, or where the cover may interfere with or complicate the automated filling of the receptacle with the end product.
Thus, it would be desirable to be able to produce containers by the thermoforming process in which the cover and receptacle were formed separately and then joined later by hinge structures formed in the cover and receptacle during the thermoforming operation. However, the thermoforming process is not well suited to the formation of structures of the type that would be analogous to the hinge pins and pivots that are utilized in, for example, reclosable injection molded containers. Because the thermoforming process utilizes a plastic sheet which has been softened by heating (but is not a liquid), the process is not well suited to form small projections or other structures that are sharply defined and have relatively small dimensions.
In accordance with the invention, a reclosable thermoformed hinged container has a separate cover and receptacle which are both produced by the thermoforming process in an efficient and economical manner. If desired, the cover and receptacle can be made of different plastic materials, of different gauge materials, in different colors and with different finishes. The separate cover and receptacle are well suited for compact shipping from the point of production of the container to the point of assembly where the receptacles are filled with the end product, since the separate cover and receptacle can be nestably stacked to provide a very compact product for shipment with minimal dead air space. The separate forming of the cover from the receptacle allows the cover to be formed with graphical material embossed therein utilizing processing conditions (e.g., heating temperature, dwell times in the mold, etc.) different from that required for forming the receptacle, and the cover is well suited to being decorated before it is assembled to the receptacle.
In accordance with the invention, hinge structures are formed in the cover and receptacle during the thermoforming process in which the cover and receptacle are separately produced. These hinge structures are formed in a manner which does not require disruption or modification of the normal thermoforming process sequence. The receptacle has an open top and is formed of a thermoformed sheet of plastic material having a hinge edge at its top, a hinge flange extending outwardly from the hinge edge, side skirt walls extending from the hinge flange generally perpendicular to the hinge edge, and an indentation formed in each side skirt wall. The cover is formed of a thermoformed sheet of plastic material and has a hinge edge, a hinge flange extending outwardly from the hinge edge, side skirt walls extending from the hinge flange generally perpendicular to the hinge edge, and an indentation formed in each of the side skirt walls of the cover. The indentations are formed in the side skirt walls of each of the receptacle and the cover along an axis of rotation. The indentations on one of the cover or the receptacle form hinge pins which, when the cover is assembled onto the receptacle, seat in the indentations in the side skirt walls of the other of the cover or receptacle to define a hinge at the axis of rotation. Assembly of the cover to the base receptacle can be easily carried out by pressing the cover onto the receptacle until the hinge pins snap fit and seat into the indentations.
In a preferred construction for the container, the hinge flange on the cover is formed to fit over the hinge flange on the receptacle, with the side skirt walls of the cover extending down over and adjacent to the side skirt walls of the receptacle. The indentations in the side skirt walls of the cover form hinge pins which extend inwardly toward one another along the axis of rotation. These hinge pins seat in sockets defined by inwardly formed indentations formed in the side skirt walls of the receptacle. An outer skirt wall may descend from the hinge flange on the cover and be integrally joined to the side skirt walls of the cover. An outer skirt wall preferably also descends from the hinge flange of the receptacle and is integrally joined with the side skirt walls of the receptacle. The receptacle may include a rim extending around the perimeter of the open top of the receptacle except where the hinge flange extends from the hinge edge, with the rim preferably comprising an outwardly extending flange and a skirt wall that extends downwardly from the outwardly extending flange and that is formed integrally therewith. The rim structure provides a smoothly formed top for the receptacle and rigidifies the structure of the receptacle. Latch structures may be formed at the front of the cover and receptacle which engage with one another to hold the cover in its closed position until opened by the user.
A particular advantage of the present invention is that the cover and receptacle may be formed of plastics, such as polystyrene, which are not well suited to use in containers having integral living hinges. The present invention thus allows containers to be formed of such plastics using the highly efficient and economical thermoforming process rather than more expensive processes, such as injection molding.
In the process for forming the receptacle or cover in accordance with the invention, a sheet of thermoplastic material of which the container is to be made is advanced over a thermoforming mold in conventional thermoforming equipment. Heat is then applied to the sheet material to heat it above its plastic transition temperature, and the softened plastic is then drawn by vacuum into the mold to conform the plastic material to the surfaces of the mold. The surfaces of the mold define the structure of the receptacle or cover including the hinge flange and the side skirt walls extending therefrom. As the sheet material in the softened state is drawn into the mold, forming pins engage the sheet material in the side skirt walls to form indentations therein. After the sheet material has cooled and hardened, the forming pins are withdrawn as the shaped product defined by the surfaces of the mold is removed from the mold. The forming pins then are advanced back into position where they can engage another sheet of material drawn into the mold. The forming pins may each be mounted on articulated forming tools which rotate about a pivot point from a position in which the forming pins are withdrawn below the adjacent surfaces of the mold to a position in which the forming pins are advanced beyond the surfaces of the mold to engage into the softened plastic sheet material of the side skirt walls. After the indentations have been formed and the sheet material has cooled and hardened, the forming tools rotate to withdraw the forming pins below the surface of the mold as the formed plastic part is withdrawn from the mold, allowing the formed part to be removed from the mold without interference from the forming pins. The process requires no change in the normal thermoforming processing steps and no significant additional processing time for the production of the formed plastic parts.